There are currently in use two basic designs in two stroke cycle engines, loop-scavenged engines with flat-top pistons, and cross-flow engines having a guide, or deflector, on the piston crown or face.
In loop-scavenged engines, gas flow in the cylinder is controlled and directed by the arrangement, shape and inclination of the transfer passages and the inlet and outlet ports. Generally, the fresh charge entering the cylinder is directed towards the cylinder wall opposite the exhaust port, thereby avoiding to some degree loss of fresh charge through the exhaust port.
The effectiveness of this system depends not only on the correct arrangement of the transfer passages and ports but to a large degree on the stability of the gas streams entering the cylinder. At low engine speeds (small throttle opening) the pressure differential in the crankcase is low, and therefore the force behind the gas streams moving through the transfer passages is low. The result is a loss of stability of the gas streams entering the cylinder. Due to this instability, the desired gas flow pattern in the cylinder may not be achieved and some loss of fresh charge through the exhaust port is inevitable. Fuel consumption at low engine speeds is high. Idling is irregular and there may be four-stroking.
Production costs of loop-scavenged engines are high due to the complexity of casting cylinders with the required arrangement of the transfer passages.
In cross-flow engines the transfer or inlet ports are placed opposite the exhaust port. Gas streams entering the cylinder through the transfer ports, upon meeting a deflector, guide, or baffle on the piston crown, are deflected upwardly, that is, away from the crown and toward the combustion chamber. In the usual cross-flow engine this may not result in efficient scavenging or filling of the cylinder. In addition to the direction of the gas flow being towards the exhaust port, a low-pressure zone at this port tends to pull a considerable amount of fresh charge over and across the deflector into the exhaust port instead of permitting the gas stream to move upwardly and along the cylinder wall portion which lies opposite the exhaust port. A particular advantage of the crossflow design is the simplicity of the port arrangement. Production costs are low since the design of the cylinder lends itself to the application of high-pressure die-cast methods of manufacture.
As used herein, the term "upward" is intended to mean the direction perpendicularly away from the piston crown toward the closed end of the cylinder and term "horizontal" is intended to mean parallel to the piston crown, without thereby implying that the cylinder should be disposed in an upright position.
Certain improvements and arrangements of the deflectors or guides, ports, and transfer passages for two stroke cycle engines are described in my U.S. Pat. No. 3,494,335 which provide good results with respect to the gas flow characteristics. It was found that the two inlet port arrangements shown in that patent, wherein the two incoming gas streams were directed in generally horizontal directions toward each other, and wherein the streams intersected between the ports, could be improved by the addition of a third inlet port located between such two ports as shown in FIGS. 6 and 7 thereof. The transfer passage for the intermediate port was arranged to provide an upwardly directed gas stream, that is, a stream directed away from the piston face toward the combustion chamber portion of the cylinder at the closed end of the cylinder, thus to increase the upward component of, or to lift, the fresh charge away from the piston face.
The intersection of the generally oppositely flowing gas streams without the intermediate port resulted in substantial turbulence, and, when a stream from such intermediate port was directed into such intersection, there was a further tendency to disturb the flow pattern of the two main gas streams resulting in no effective reduction of the undesired turbulence.
Another approach to the problems of efficiently scavenging two stroke cycle engines is represented by French Pat. No. 775,698 issued in 1934 to Cycles Peugeot, wherein a T-shaped deflector or guide is provided on the piston crown. A first part of the guide extends laterally across the piston crown separating the inlet ports from the exhaust port and a second leg flange part extends from the center of the first part toward the piston wall portion which lies between the inlet ports. Two more or less opposed gas streams enter the cylinder generally horizontally to be guided along the first laterally extending part of the deflector toward the second leg flange part, which part then deflects the gas streams upwardly toward the combustion chamber.
The arrangement of the transfer or inlet ports of the Peugeot engine undesirably restricts the effective cross-sectional port area that can be obtained, and this limits the specific power output.